Cold Harbor, Va., Vicinity. Ruins of Gaines' Mill refers to a historical site associated with the American Civil War, specifically the Battle of Gaines' Mill that took place on June 27, 1862, during the Peninsula Campaign. This battlefield is located in present-day Hanover County, Virginia. The Ruins of Gaines' Mill monument marks an area where intense fighting occurred between Union and Confederate forces.The Battle of Gaines' Mill was a significant engagement that ended inconclusively, resulting in high casualties for both sides. Despite initial successes by the Union Army under General George McClellan, the Confederates ultimately repelled their advance, leading to a stalemate. The Ruins of Gaines' Mill monument serves as a lasting reminder of this important event in American history and the sacrifices made during the U.S. Civil War.